1. Field of the Invention
The present invention relates to a fabrication method for a waveguide, and more particularly, to a method of fabricating periodic domain inversion regions within a ferroelectric single crystal substrate by employing an etching process.
2. Background of the Related Art
Many researches have been conducted on quasi-phase-matching wavelength conversion devices employing a ferroelectric substrate having a non-linear characteristic for a long time. Important application fields employing this technique can include displays, light sources for the next-generation recording media, optical communication systems and so on. In particular, in case where quasi-phase matching is implemented through periodic inversion of domains within these ferroelectric crystals, high efficiency characteristics can be obtained. In general, the inverted domains are fabricated in a periodic polarization form. The domain fabricated by this method is referred to as a periodic polarization inversion ferroelectric substance.
In a periodic polarization process, important factors which affect devices can include regularity of the polarization-inverted domain cycle and accurate control of the inversion cycle interval. These factors decide the quality or efficiency, and the range of applications of fabricated devices.
For this reason, a variety of methods for forming the periodic polarization inversion domains within the ferroelectric substance have been developed, and the principles thereof are classified depending on a change in the shape of an electrode and the type of external force for domain inversion.
In particular, among these methods, an external electric field application method has been used as a representative one. Further, many researches have been conducted on light-assisted poling (hereinafter, referred to as “LAP”) in which light and an electric field are irradiated at the same time, all optical poling (hereinafter, referred to as “AOP”) in which only light is used without applying an external electric field, and so on.
FIG. 1a shows a periodic polarization inversion method based on an electric field application method employing z-cut lithium niobate (LiNbO3) crystals. In order to form periodic domains within a Z-axis ferroelectric substrate 100, conductive layers 110 have to be formed on the crystal surface so that an external electric field can be applied to the crystals. The shape of the conductive layers becomes a domain inversion shape, and domain inversion always proceeds from a positive (+) plane to a negative (−) plane. Thus, the conductive layers have to be formed on the positive (+) plane. The conductive layers can be fabricated by two kinds of methods. One method includes directly forming a metal pattern on the crystal surface. The other method includes forming a pattern on the crystal surface by using organic substance such as a photosensitizer, and then forming an opened space so that an external electric field can be applied to the crystals through conductive liquid.
Therefore, after patterns for domain inversion are formed on a ferroelectric Z-axis positive (+) plane, an external electric field is applied to the +Z plane, thus forming a periodic inversion structure. For the purpose of domain inversion of Z-axis crystals, a coercive field of 21 KV/mm or higher is required. In this case, a periodic polarization structure can be obtained over the entire regions ranging from the +Z plane to the −Z plane.
FIG. 1b shows a periodic polarization inversion method based on an electric field application method employing x-cut LiNbO3 crystals. Domain inversion always proceeds in the Z-axis. Thus, a straight conductive layer 130 is formed one side on a substrate 120 of an x-axis crystal so that it is vertical to the Z-axis. Further, branch-shaped conductive layers 140, which are spaced apart at a certain distance, have to be formed on the other side on the substrate 120.
Consequently, polarization is inverted by a mutual electric field between the branch-shaped conductive layers and the straight conductive layer, and a distance between the widths of the branch-shaped conductive layers becomes a periodic domain inversion cycle. In case where periodic polarization inversion is performed by employing x-axis ferroelectric crystals, there is an advantage in that coercive voltage is relatively small compared with domain inversion of Z-axis crystals since a distance between electrodes for polarization inversion is short.
In addition, LAP in which both light and an electric field are irradiated is a method of inverting domains by irradiating light of wavelengths of 457 nm, 488 nm, and 514 nm under a uniform external electric field. In this case, a pattern of the light becomes a domain pattern. Furthermore, AOP employing only light is a method of inverting domains by irradiating only light without an external electric field. In this case, a pattern of the light becomes a domain pattern.
However, the above methods have lots of problems, such as a low domain depth, irregularity of domains, and control of domains formed, and therefore cannot be commercialized.
In the conventional external electric field application method for periodic domain inversion, if the Z-axis substrate is employed, the highest voltage is applied to the edges of the electrode pattern formed on the substrate, so that initial domain nucleation for domain inversion is formed. Thus, domain tips are created in a direction vertical to the −z side along the nucleation formed at the electrode edges of the +z plane. Further, an external electric field through the electrode is applied within the electrode as well as the electrode edges. It has an effect on the domains inwardly and outwardly from the electrode. Thus, domain walls, which are collections of the domain tips, proceed in both directions on the basis of the edges of the electrode. The progress of the domain walls is stopped when they are combined within the electrode. Accordingly, the formed periodic domains have the following problems.
First, since the domains are inverted through the whole crystal substrate, they have a high aspect ratio, so that a domain cycle that can be fabricated is limited.
Second, there is a difference between a pattern on a substrate for domain formation and a domain cycle that is substantially formed. Thus, control is difficult as the cycle is short.
Third, a coercive voltage of about 21 KV/mm or higher is required for the purpose of domain inversion.
Fourth, domain quality is limited due to irregularity of an external voltage.
Meanwhile, if a ferroelectric substrate x-axis is employed, there is an advantage in that a coercive voltage is low because of a short distance between electrodes. However, as polarization is inverted by a voltage applied to both electrodes, there are disadvantages in that a domain depth is shallow, and the regularity of a domain cycle is poor when compared with a method of employing Z-axis crystals.